We study the low-energy excitations of the spin-1/2 antiferromagnetic Heisenberg chain and N-leg (N = 2,3,4) ladders in a staggered magnetic field h(s). We show that hs induces gap and midgap states in all the cases and we will examine their field scaling behavior. A modified boundary scheme is devised to extract accurate bulk excitation behavior. The gap values converge rapidly as N increases, leading to a field scaling exponent gamma=1/2 for both the longitudinal and transverse gaps of the square lattice (N ->infinity). The midgap states induced by the boundary edge effects share the bulk gap scaling exponents but their overall scaling behavior in the large-N limit needs further investigation.